Energy: what is it?
The term 'energy' is quite fashionable today among the proponents of many different arts and beliefs. From alternative medicines, to various mysticisms and religions, we can see terms like 'cosmic energies', 'energy fields', 'positive/negative energies', 'blocked energy' and many more.
But what is energy after all?
The most basic definition in physics is essentially the capacity for work. This may not seem much help, but it does indicate that energy is not a 'thing' in itself, but rather a property of something else.
To understand this better, let's look at an analogous case: velocity. Everyone knows what velocity is -- the change in position at each instant -- and velocity is also a property of a system, not an independent entity.
When you step on the gas pedal, the velocity of your car increases. When you brake, the velocity decreases. The velocity of the car changes but it does not sound right to say that velocity flows into your car when you accelerate or that it leaks from your car when you brake. Velocity is a property of your car that can change, not something that flies around waiting to be used.
Exactly the same thing happens with energy. When you light a match, the burning materials change and the energy of this system decreases (like the velocity of the breaking car) and the energy of other systems increases, such as the air surrounding the match or anything else the match heats or lights. Energy is not something that actually 'passes' from one system to the other. What happens is just that when the energy in one system decreases the energy in some other system must increase.
The big difference between energy and velocity is that energy is conserved. When the energy in one system decreases the energy of some other system must increase by exactly the same amount. This makes it natural, and sometimes even useful, to picture energy as something jumping from one place to another. But this is not an accurate picture of what happens. To illustrate, let's look again at velocity, but this time under conditions where velocity too is conserved.
Imagine that you are drifting through space towards a space station at 30mph, and that you brought from Earth a rock of your weight. If you shove the rock so that it starts travelling towards the space station at 40mph (10mph faster than you), your own speed will drop to 20mph. The total velocity remains the same, but instead of both you and the rock moving at 30mph, now you are moving at 20mph and the rock at 40mph. We can picture that 10mph of velocity was passed from you to the rock, but this is a risky analogy because it gives the impression that velocity is something you can carry around or pass from one thing to another.
The same problem occurs with the picture of energy being transferred from some system to another system. It is useful to remind us that the total energy must be conserved (i.e. the total change over all systems must be zero), but it cannot be taken beyond that. Like velocity, energy is a property, not a 'thing'.
Nevertheless, many people use the word 'energy' as if it referred to some substance. So in acupuncture you can remove the blocks that stop energy from 'flowing' in your body. Touch therapists correct the 'energy field' around you where 'negative energies' may allegedly accumulate, and several herbs are used to 'balance your energies'.
What this means has nothing to do with the scientific usage of energy. In these cases 'energy' is simply a way of saying 'spirit' or 'soul' with a more scientific ring, or just referring to sensations like warmth or well being.
Another thing to remember is that energy, like velocity, is a quantifiable property. It's a bit pointless to tell you that my car is moving with a velocity if I don't tell you what that velocity is, such as twice as fast as your car or 60mph. Similarly, it doesn't help much to say that a process involves energy if there is no indication of the magnitude. A burning match and an atomic explosion both involve energy changes, but they are very different in how big the change is. In most cases where the word 'energy' is misused, you will see no mention as to how much energy that is.
Whenever you see the term energy being used, remember that energy is a property of something else, like velocity, not some substance or entity. There are no different kinds of velocity, and no different kinds of energy. There are no things made out of velocity, nor things made out of energy. And, just like velocity, simply saying that something has energy is not useful if no there is no idea of how much energy is involved.
Many thanks to the members of the Skeptic list at firstname.lastname@example.org for their help in the preparation of this document.
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